1. Field of the Invention
This invention relates to a portable air-blowing working machine having a prime mover and a centrifugal air blower adapted to be rotatably driven by the prime mover. More specifically, the invention relates to such a portable air-blowing machine in which the centrifugal air-blower is embodied in two stages, which stages share a common unitary double fan unit.
2. The Prior Art
Portable air-blowing working machines of the aforementioned type may, for instance, comprise portable air-blowing cleaners for collecting scattered debris, such as fallen leaves, dust and the like, or portable spraying machines for spraying chemicals.
FIG. 6 shows one example of a portable air-blowing cleaner of the aforementioned type of portable air-blowing working machine. The portable air-blowing cleaner 100 shown in FIG. 6 is of the backpack type and is designed to produce a high velocity, compressed air stream so as to "sweep" and/or gather scattered debris, such as fallen leaves, dust and the like. Such a portable air-blowing cleaner 100, therefore, can advantageously be used in lieu of a conventional broom (See U.S. Pat. No. 5,052,073). As illustrated, the portable air-blowing cleaner 100 includes a U-shaped (in plan view) shouldering frame 12 to which a pair of shouldering straps 14 are connected, a centrifugal air blower 80 which is attached as an air-blowing system to the shouldering frame 12 via a vibration-damping pad (not shown), and an air-cooled two-stroke gasoline engine 20 which is directly attached as a prime mover behind the air blower 80 for rotatably driving the air blower 80.
The centrifugal air blower 80 is of the conventional single-stage type, and is designed to draw in external air, increase it in velocity and pressure, and discharge it as a high velocity air stream through a discharge port 85 that is formed on one side of the air blower 80 so as to extend obliquely downward. To this air-discharge port 85 are successively connected a bent pipe 91, a bellows type flexible pipe 92 and an exhaust pipe 95. On the upstream side of the exhaust pipe 95, there is mounted an operating handle 96 which is provided with manipulating members 97, such as a throttle valve control lever and a switch, for controlling the speed of the engine 20.
As mentioned above, according to the conventional portable air-blowing cleaner, a centrifugal air blower of the single-stage type has been employed as the air-blowing system. Recently, however, the noise generated by such conventional air-blowing cleaners has become a noise pollution problem. In some regions (California U.S.A, etc.) the enactment of regulations to prohibit the sale of such portable air-blowing working machine has been discussed.
Various countermeasures have been proposed to minimize the noise generated by conventional portable air-blowing working machines. One proposal is to cover the air blower entirely with a covering member lined with a noise-damping material. All of the countermeasures that have been proposed to date, however, have failed satisfactorily to minimize the noise output and/or have been accompanied by a degradation of desirable features of the portable air-blowing working machine, such as its light weight and compactness in size.